1. Field of the Invention
The present invention relates to the field of image sensors in use for facsimile or the like equipment, and in paticular, to the field of elongated image sensors now under development instead of IC type image sensors such as MOS photodiode arrays or CCD image sensors.
2. Description of the Prior Art
An elongated image sensor is so constructed that a photoelectric conversion array comprising a plurality of photoelectric conversion elements and a circuit for sequentially switching those elements are formed on a single insulative substrate, and is designed such that the width of the photoelectric conversion element array is sized the same as that of an original. The image on the original is read in the one-to-one image forming manner by an optical system such as an optical fiber array or a lens array. As a result, the optical path length for focusing the image can be shortened, thus enabling the reduction in size of an image reading apparatus.
FIG. 1 shows a circuit diagram of a prior art elongated image sensor, in which the image sensor 1 comprises photoelectric conversion elements 10 of a photoconductive film respectively represented equivalently by a photodiode PD and a capacitor C, switch elements 20 for picking up a signal generated at the photo-electric conversion element 10, capacitance 30 comprised of a parasitic capacitance formed by stray capacitance of the wiring connecting the photoelectric conversion element 10 and the switch element 20 and an input capacitance of the switch element 20, a charge transfer circuit 40, and a load resistor 50. Reference numeral 60 indicates a power supply for biasing. The charge transfer circuit 40 is comprised of charge storage capacitance 41, charge transfer switch elements 42, and clock lines 43 and 44.
When an image on the original is focused on the photoelectric conversion element 10, a photocurrent is generated at the photodiode PD corresponding to the light intensity, and a signal charge is thus stored in capacitance 30. When the switch element 20 is closed by applying a suitable signal to a terminal T1, the signal charge stored in the capacitance 30 is fed to the charge storage capacitance 41. Then as the charge transfer switch elements 42 are sequentially actuated by an alternating voltage applied via terminals T2 and T3 to the clock lines 43 and 44, the signal charge stored in the charge storage capacitance 41 is shifted to the right-hand direction of FIG. 1, eventually applied to the load resistor 50, and outputted through an output terminal T4 as a voltage signal.
As the switch element 20 and the charge transfer switch element 42, MOS transistors produced by ordinary IC technology may be used. The charge transfer circuit 4 of this case is BBD (bucket brigade device).
A MOS transistor is suitable as a switch element of the image sensor due to its features of comparatively high switching speed and the capability of being operated at a low voltage. For realizing the above-mentioned elongated image sensor, it is necessary to mount the switch elements 10 and an IC chip on which the switch elements 20 and the charge transfer circuit 40 are formed on a single substrate and further to connect the above two with each other by such means as wire bonding, causing the increase in the number of connection lines for each image sensor. This fact is a serious problem in producing highly reliable devices at low cost.
As a means for solving the aforementioned problem, it has been proposed that the switch element 20 and the charge transfer circuit 40 be formed of thin film transistors employing semiconductor film. According to this method, since the switch element 20 and the charge transfer circuit 40 are formed by the same film forming process as that for the photoelectric conversion element 10, the above-mentioned connection related problem can be solved favorably.
However, because of slow switching speed compared with the MOS transistor and high voltage required for the actuation of switching operation, the thin film transistor is not satisfactory as a switching element to be applied to such an image sensor, leaving the method imperfect for practical application.